A number of semiconductor applications require the formation of a cadmium telluride (CdTe) layer over a silicon (Si) substrate. For example, a larger CdTe substrate can be formed in this manner. While silicon can be formed in substrates of eight to twelve inch diameters or more, CdTe substrates are typically sized at about 4 cm.times.6 cm. Therefore, an increase of more than thirteen times can be achieved if the CdTe can be grown on an eight-inch silicon wafer.
Another application which requires cadmium telluride layers formed on silicon substrates is found in infrared detectors. Cryogenic infrared detectors are typically made of small band gap (about 0.1-0.2 eV) semiconductors such as HgCdTe (mercury cadmium telluride) grown on CdTe (with approx. 4% ZnTe added for lattice matching to HgCdTe) substrates and operate as photodiodes or photoconductors by photo absorption to produce electron-hole pairs. For example, U.S. Pat. No. 4,686,373 describes a hybrid system with HgCdTe photoconductors bonded to silicon signal processing circuitry.
A problem arises because of the huge thermal mismatch between the silicon and the mercury cadmium telluride. One way to overcome this thermal mismatch problem is to provide a second silicon layer formed on the other side of the HgCdTe substrate so that the HgCdTe is sandwiched between the two silicon layers. In this manner, the thermal mismatches will cancel.
One way to form this layer is to epitaxially grow cadmium telluride (CdTe) on a silicon wafer. For example, R Sporken et al., Molecular beam epitaxial growth of CdTe and HgCdTe on Si (100), Appl. Phys. Lett. 55(18) (1989), p. 1879, teaches a method of forming a CdTe layer on a silicon substrate. A HgCdTe layer can then be grown upon the CdTe layer by molecular beam epitaxy (MBE), metalorganic chemical vapor deposition (MOCVD), or liquid phase epitaxy (LPE).
However, a problem exists because of the lattice mismatch and crystal structure between the cadmium telluride and the silicon. For example, direct growth of cadmium telluride on silicon (100) using MBE or MOCVD gives cadmium telluride (111) on silicon (100) leading to a non-parallel epitaxy. This non-parallel epitaxy is undesirable and therefore a method of growing cadmium telluride (100) upon silicon is presently desired, that can be applied to other orientations such as (211) to maintain parallel epitaxy.
Other techniques have utilized ZnTe as a buffer layer between Si and CdZnTe. See e.g., T. J. de Lyon et al., Direct molecular-beam epitaxial growth of ZnTe (100) and CdZnTe (100)/ZnTe (100) on Si (100) substrates, Appl. Phys. Lett., 63 (Aug. 9, 1993), p. 818. In this case, single-crystal CdZnTe (100) films were grown on Si with the use of ZnTe buffer layers.